Re: New to multiboxing...
Running all 4 D3 clients on a single monitor stacked, at 1920x1080, with all settings set to low seems to work fine on a single card. During large fights I did get a little lag, but it was still playable.
Idle aka, sitting in town the single card runs at about 75%, when fighting it pretty much stays pegged at 100%.
Hope this helps.